A major impediment to recovery after spinal cord injury (SCI), traumatic brain injury (TBI), or stroke is the failure of central nervous system (CMS) axons to regenerate effectively through white mater over long distances. A variety of factors are believed to contribute to this problem. These include inhibitors in glial scars, inhibitory material associated with myelin or damaged myelin and molecular changes in neurons during development that reduce their potential for axon growth. Over the past several years it has been shown that Dorsal Root Ganglion (DRG) neurons can send axons very long distances in white mater if they are transplanted into the CNS using techniques that minimize damage. In contrast, transplanting CNS neurons the same way does not produce the same result;i.e. they fail to send out long axons through white matter tracks. This implies that DRG neurons and CNS neurons have inherent molecular differences that limit CNS regenerative efficiency. We propose to test a specific hypothesis that DRG neurons express different genes than CNS neurons, which permit DRG neurons to regeneration in the CNS.
In specific aim 1 we will identify these molecular differences using serial subtraction of cDNA libraries. We will look for unique genes in DRG neurons that might enhance regeneration and unique genes in hippocampal neurons and corticospinal neurons that might inhibit regeneration This method is extremely effective at identifying rare and perhaps novel cDNAs, ensuring identification of important targets, such as transcription factors. We will also search public microarray databases to search for additional candidates.
In specific aim 2 candidate genes will be tested using a well-established in vitro assay where neurons are grown on myelin or proteoglycans and the lengths of their neurites measured. CNS neurons will be transfected with DRG specific genes or use RNAi of CNS specific genes in order to evaluate the target genes roles in axon growth on inhibitory substrates. Specific aim 3 will use neuronal transfection and microtransplantation in vivo. This will permit us to directly test the role of each candidate gene in the most relevant assay, regeneration in the mammalian central nervous system. These experiments will provide entirely new information about the proteins expressed in DRG neurons that allows them to extend long axons in the CNS. The identification of these targets and testing multiple candidates using in vitro methods and subsequently a refined subset using in vivo approaches should lead directly to potential treatments for SCI, TBI and stroke. Lay Summary: These experiments are designed to identify genes involved in controlling regeneration in white matter in the adult brain. The genes will be tested in neurons that cannot normally grow axons using cell culture and transplantation into animals to determine if the genes promote regeneration.